This invention relates to improvements in sound reproduction, and particularly to an improved loudspeaker system for providing improved ambient imaging, as well as improved image location, without regard to listener position relative to a pair of said loudspeaker systems which are spaced apart and located generally facing the listener.
Loudspeaker systems generally comprise one or more drivers or driver elements (sometimes referred to as speakers), which are arranged within a common cabinet, enclosure or other housing or mounting structure in such a way as to provide a full range of audio frequencies. The drivers or speakers are also often arranged so as to radiate these audio frequencies in a given pattern. In the present description, the term loudspeaker or loudspeaker system will be utilized to refer to an apparatus in which one or more drivers are mounted, arrayed and electrically connected for radiating audio signals in a desired pattern. The term "driver" will be used to refer to the individual "speaker" elements which may be arrayed or mounted in such a loudspeaker system. As such, a driver commonly includes at least one radiating element such as a cone, horn, dome or the like and an electrically driven element such as a voice coil which may contain one or more windings for applying electrical signals thereto, which signals in turn drive the cone or other sound radiating element. In this way, the electrical signals fed to the voice coil or other driven elements are reproduced as sound vibrations by the cone or other radiating element.
Generally speaking, stereophonic sound recordings or broadcasts rely on the spacing of microphones during recording or broadcast for their dimensional content. When respective right and left channel microphones are used in the recording or broadcast, the reproducing of the same left and right information in separate left and right loudspeaker systems can be used to reproduce compatible spatial or ambient characteristics as in the original location of the recording or broadcast. That is, the phase and amplitude differences between what is recorded or reproduced on the left versus the right can cause the ear/brain mechanism to interpret the resulting sound reproduction as having a spatial reality in spite of contributions of the listening room. In customary stereo sound recording and reproduction, respective left and right channels are intended to reproduce corresponding information in the original recording or broadcast such that verbatim physical conditions are not required in the listening room for the ear/brain combination to selectively "tune out" the listening room's time and amplitude information and instead "hear" the spatial and ambience characteristics of the original recorded or broadcast event.
However, for the listener's ear/brain mechanism to be convinced of the spatial ambience of the program, it is necessary that the left and right channel information reach the listener's left and right ears both independently and in a time sequence consistent with the original recording ambient "signature". The impinging listening room ambient "signature" makes the listener instantly aware that he is listening to a reproduction of an original acoustic event and not the event itself.
In addition, acoustic artifacts unique to loudspeakers in rooms create dimensional compression in the lateral plane further diminishing the credibility of the experience. Dimensional compression means that acoustic events occuring at right angles to a listener situated in a concert hall are perceived through loudspeakers as though they were squeezed between the two loudspeakers.
Typically, listeners attempt to regain some of the dimensional impact of the original performance by spacing their loudspeakers as far apart as possible. When this is done the central image and specificity are lost. Individual performers seem to be split into left and right loudspeakers.
The two principal elements in lateral localization of sound are time (phase) and intensity. A louder sound seems closer and a sound arriving later in time seems farther away. The listener will employ the two ears and the perceptive interval between the two ears to establish lateral location. This involves the Pinnar effect, often discussed in terms of "interaural cross-correlation".
In typical prior art speaker system arrangements, a listener is positioned in front of and equidistant from equivolume radiating speakers of a pair of loudspeaker systems. The respective left and right loudspeaker systems produce the right and left stereo channels essentially monophonically. Whatever dimensional quality exists in the experience is created when the listener (who must be equidistant from the left and right loudspeakers) is able to compare the left and right acoustic events through the ear/brain mechanism. It should be noted that this "difference component" exists only between the two loudspeakers and diminishes as the listener approaches either loudspeaker. Independent right ear and left ear perception may be compromised by some left ear perception of the right channel and vice versa. The perception of these interaural effects is in the early arrival time domain, so that the later arrival room reflections do not ameliorate the diminished perceptions of the left and right difference components.
Thus, as a listener moves into a position closer to one loudspeaker system than the other, the effect worsens. The output from the more distant loudspeaker system appears reduced until sound from only the nearer system envelops the listener. Since the stereophonic effect of the recorded or broadcast material depends on the listener's perception of the difference between channels, the reduction of either of these channels tends to further compromise the already interaurally compromised left-right signal.
Accordingly, some listeners have expressed a preference for sound reproduced through stereophonic headphones. These isolate the recorded, time-related ambience by blocking out the impinging ambience of the listening room. They also prevent the slightly delayed arrival of left information at the right ear and vice versa. However, headphones provide limited acoustic performance, principally in the bass region, and generally are confining and somewhat uncomfortable to wear for prolonged periods.
While it may be possible to affect the ear/brain mechanism localizing ability by employing artificial clues such as deliberate phase shifts, the prior art has generally not employed such techniques because no universally accepted first-order model exists. Prior art loudspeakers ignoring both room interaction and recording techniques have concentrated on the loudspeaker as a straight forward power device. Design efforts have been directed at providing the most uniform total radiated power response with a minimum of transducer generated distortion components. This insures that the perceived output may have accurate instrumental timbre, but does not insure that the listener will hear a dimensionally convincing version of the original sound from a wide range of positions in a normal listening room. For most stereophonic reproduction devices, the stereo signals are typically reproduced by a loudspeaker system or apparatus that incorporates a plurality of drivers, usually of different frequency characteristics, mounted in spatially fixed relation to one another in an enclosure. Typically two such enclosures are utilized, the drivers of each enclosure wired to reproduce only the left channel or only the right channel stereophonic signals.
In the past several years, however, efforts have been made to overcome some of the acoustic problems with these types of loudspeaker systems, as discussed above. A number of other prior art patents and their approaches are discussed in the inventor's own prior U.S. Pat. No. 4,847,904, issued Jul. 11, 1989, to which reference is invited.
The aforesaid prior U.S. Pat. No. 4,847,904 teaches an arrangement of drivers in a loudspeaker system including at least one dual-coil driver in each speaker system connected so as to reproduce only the difference between the left and right channels. This patent additionally teaches providing respectively inwardly and outwardly facing drivers in each of respective left and right speaker systems and connecting their respective outwardly facing drivers to produce the left-right difference signals.
In addition, specific polarity relationships between the L-R and R-L outwardly radiating drivers and their respective L and R inward radiating drivers are employed in my prior patent. Specifically, if the L component of the left speaker system inwardly radiating element is deliberately connected in opposing phase to the L component of the R-L outwardly radiating element of the left speaker system, and if the same is done with the R components of the right speaker system, a narrow angle acoustic null is created in the area immediately in front of each speaker system. The width of the null is determined by the precise relative angle of the two drivers and their spacing.
Accordingly, as a listener moves directly in front of either the left or right speaker, the opposite or more distant speaker tends to sound louder. By providing respective difference components (L-R and R-L) on the outboard facing driver of each speaker system, each ear also tends to reject information that normally would travel to the opposite ear from the same channel. The effect is somewhat like wearing headphones or building an acoustic wall to isolate the left and right channels from the respective opposite ears. Hence, in the arrangement taught by my prior patent, the listener in almost any position in the listening room will sense either from direct output from a difference driver, or from reflected images created by room walls, significantly enhanced ambient information.
The nature of the difference (L-R and R-L) components is to emphasize late arrival reflected information. In my prior patent, the relative gain of the inward-outward radiating drivers can be considered infinitely adjustable. However, as the relative gain of the difference drivers is increased relative to the inward L mono or R mono image, specificity can be compromised.
I have now determined, however, that a further improvement in the imaging can be obtained by maintaining the same polarity as between the like components of the inwardly and outwardly radiating drivers of the respective speaker systems. That is, rather than connecting the left component of the R-L element in opposing phase to the left inwardly radiating element, the two should be maintained in the same phase relation. Similarly, instead of connecting the right component of the L-R outwardly radiating element in opposing phase to the right inwardly radiating element of the right speaker system, these two right components should also be connected in phase.
I have further determined that in order to avoid an overly vague central image, the inwardly facing driver of each speaker system, physically arranged generally as taught in my aforementioned prior patent, can provide monophonic information in addition to its normal left or right channel function. This can be done by using a dual voice coil connected L+R or R+L. Such a speaker connected to a stereophonic source will produce only a composite monophonic component. This "in-phase" signal L+R, R+L, tends to narrow the sound stage and produce a more sharply defined center image.
In U.S. Pat. No. 4,748,669 to Klayman, stereo sum and difference components are extensively processed and manipulated in both recording and sound reproduction applications. However, the extensive processing, external additional circuitry and cross-channel amplifier wiring taught by this patent may degrade the audio signal with hum or noise.
In Polk U.S. Pat. No. 4,569,074, a speaker system is employed in which two drivers face in the same direction such that the axes along which the two drivers radiate sound are essentially parallel. These drivers are arranged essentially side by side such that one may be referred to as inboard and the other as outboard. These two drivers are coupled such that in the left system the inboard driver produces only the left stereo component, while the outboard driver produces a left-minus-right (L-R) difference component. In the right speaker system, the inboard driver produces only the right stereo component, while the outboard driver produces an R-L component. While this system produces good imaging for a listener positioned equidistant from the two speaker systems, it fails to produce a satisfactory image for a listener in any other position relative to the speaker systems.